Nonvolatile, resistively switching memory cells include a first electrode, a second electrode and a solid electrolyte that is contact-connected by the two electrodes in a sandwich-type geometry. The solid electrolyte includes an amorphous matrix of a chalcogenide compound and a metal which is distributed in the amorphous matrix and the cations of which migrate in the amorphous matrix to the cathode under the influence of an electric voltage. The memory cells, on account of the migration of these cations, are switchable by a first electric voltage into a state with a low electrical resistance and by a second electric voltage of opposite polarity into a state with a high electrical resistance.
These two physical states (i.e., the state with the high electrical resistance and the state with the low electrical resistance) are used for the storage of information. Usually, the higher electrical resistance is assigned to logic 0 and the lower electrical resistance is assigned to logic 1.
After the brief supply of current has been switched off, specifically both after programming (transition to the state with the low electrical resistance) and after erasing (transition to the state with the high electrical resistance) or reading of the information, the resistance value which was induced last is retained in stable form for a prolonged period of time. Accordingly, the storage of information in resistively switching memory cells is nonvolatile.
The strong increase in the conductivity of the resistively switching memory cell when a voltage is applied is based on electrochemical processes in the solid electrolyte of the memory cell, which leads to the formation of a metallic bridge that extends all the way from the cathode to the anode. Applying a voltage of opposite polarity at least partially removes this bridge again, and the metallically conductive path between the electrodes is interrupted: the memory cell returns to the state with the high resistance. On account of this storage mechanism, nonvolatile, resistively switching memory cells are also known as nonvolatile CBRAM (conductive bridging random access memory) memory cells. This form of memory cells is distinguished by a low switching voltage with short switching times. Furthermore, with this type of memory cells, a high number of switching cycles is achieved with a good thermal stability.
The memory cell designs which have been used hitherto (such as for example floating gate memories, such as flash or DRAM), on account of their mechanism being based on the storage of charges, will reach the limits of scalability within the foreseeable future. The problem with DRAM and flash memory cells is that with ongoing miniaturization of the memory cells, the charge quantity which can be held in a cell is also becoming ever smaller. However, as the charge quantity decreases, so too does the reliability of information storage. Memory cells which are based on capacitive charging no longer operate satisfactorily, on account of the lower voltages and current intensities caused by miniaturization in this “low-energy” range.
Furthermore, as in the case of the flash memory, the high switching voltages and the limited number of read and write cycles, and in the case of the DRAM the very limited duration of storage of the charge state, constitute additional problems which it has not hitherto been possible to solve in an optimum way. The problem of leakage currents within the storage capacitor which exists with the DRAM design, leading to the loss of charge, is currently resolved to only an unsatisfactory extent by constant refreshing of the stored charge.
These and other problems have led to new storage technologies, which are based on concepts other than capacitive charging, having been developed in recent years. In this context, the nonvolatile CBRAM memory cells mentioned above are very promising. These memory cells have been described, for example, in International Patent Application Publication Nos. WO 97/48032 and WO 03/032392, U.S. Pat. No. 6,418,049, and U.S. Patent Application Publication Nos. 2003/020997 and 2003/0209728.
The storage concept of CBRAM memory cells is not based on the storage of charges, but rather on the difference in resistance between two stable states, which is caused by a high mobility of metal ions in a matrix of chalcogenide compound(s) when an electric field is applied.
As noted above, CBRAM memory cells typically include two electrodes and a thin film of a solid electrolyte that is contact-connected by the two electrodes in a sandwich-type geometry. Solid electrolytes may be crystalline or amorphous solids. The solid electrolytes used in CBRAM cells typically include amorphous solids, which can also be referred to as amorphous matrix or glasses. The solid electrolytes are preferably chalcogenide compounds.
Chalcogenides are compounds in which one or more elements of main group VI of the Periodic System (oxygen, sulfur, selenium, tellurium) form the more electronegative compound, which are referred to as oxides, sulfides, selenides and tellurides. Sulfides and selenides have a pronounced tendency to form amorphous solids. They are therefore preferably used for the fabrication of CBRAM cells. Oxides can likewise be formed as amorphous layers, but their structure (microstructure) is generally so dense that the ionic mobility of the metallic component is too low. Sulfides and in particular selenides have a more open microstructure and are therefore preferred from the perspective of switching speed in CBRAM cells.
Germanium from main group IV of the Periodic System can be used as the more electropositive chemical element in the chalcogenide compounds. These IV-VI compounds, as they are known, are suitable as amorphous matrices for CBRAM memory cells; germanium sulfide and germanium selenide represent preferred IV-VI compounds for use as amorphous matrices in the CBRAM memory cells. Alternatively, however, silicon selenide or silicon sulfide are also suitable as amorphous matrices for CBRAM memory cells.
The selenium or sulfur content in germanium selenide GexSe1-x and germanium sulfide GexS1-x may vary over a wide range. Particularly suitable glasses with ideal properties are formed if x is in the range from 0.2 to 0.5 and is, for example, 0.33. Generally, x must have a value which is such that the corresponding selenide or sulfide can easily be formed into a stable glass that is suitable for solid ion conduction. In the description which follows below, all selenide and sulfide glasses are hereinafter referred to as Ge—S and Ge—Se chalcogenide compounds, irrespective of the particular value of x.
In the amorphous matrix which forms the active material of the CRAM memory cell, the metal which is contained, under the influence of an electric field, forms cations which migrate through the amorphous matrix to the cathode under the influence of this field. Particularly suitable metals are silver (Ag) and copper (Cu), with silver being preferred, since it is easy to ionize and in the ionized state (as Ag+) has the required mobility in the amorphous matrix of the chalcogenide glass. The quantity of silver which can be taken up by Ge—Se and Ge—S compounds depends on the Ge/S or Ge/Se quantitative ratio. This saturation concentration is typically a few 10 s of atomic percent, for example Ge0.3Se0.7 is 47.3 atomic percent of silver.
The cathode is typically composed of an inert material, such as molybdenum or tungsten. The cathode may also be composed of tantalum, titanium, such as conductive oxides, nitrides, or highly n-doped or highly p-doped silicon or alloys of the abovementioned materials. As used herein, the term “inert material” means a conductive material which, under the influence of an electric field, conducts the current but does not form cations, or its cations, when an electric field is applied and has no mobility or only an insignificant mobility within the amorphous matrix.
A metal or a chemical compound whose cations are suitable for ion migration through the solid electrolyte is used as the anode. The anode is preferably composed of the metal which is also contained in the solid electrolyte. As result, the quantity of metal which is electrochemically deposited at the cathode can be topped up by the oxidation of the anode. It is therefore preferable to use an anode made from copper, silver or silver compounds, such as for example Ag2S, Ag2Se.
In the processes used hitherto for fabricating CBRAM memory cells, solid electrolytes, such as the preferred Ge—Se—Ag glasses and Ge—S—Ag glasses, have been formed by photolytic dissolution (photodiffusion) of silver in a thin film of the solid electrolyte. One drawback of these fabrication methods is that the photolytic dissolution of silver is carried out in a separate process step, generally by UV exposure, such as by mercury vapor lamps, so that in this step it is necessary for the wafers to be discharged from the vacuum chamber and then reintroduced into the vacuum chamber for further processing following the photodiffusion. As a result of this process step, there is a very high risk of contamination of the wafer and/or of the wafer being covered with foreign particles.